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Waveform generator produces biological-stimuli signals

Electrical simulators are widely used in biological research for the study of evoked potentials in man and other applications. Indeed, versions are on sale in local health stores or via mail order for treatment of pain. However, for the stimulation to be most effective, the pulse shape needs to be a certain configuration: It has to be bipolar and have only one edge, similar in form to two sawtooth waveforms back-to-back with the second inverted (Fig. 1).

This type of waveform is generally described as a Zeta waveform. Zeta waveforms can be generated very easily by using commercially available arbitrary waveform generators. But this approach can be expensive, particularly for a “one-of-a-kind” project.

Another possible solution would be to use a PROM and digital-to-analog converter. However, the best solution is to generate the waveform from analog components with little time and effort and low cost.

The circuit operates as follows (Fig. 2): U1A and U1B form a triangle waveform generator using R6 to offset the output so that it’s a positive-going signal. The amplitude is set by R2 and R3, and the ramp’s rate of rise is set by R1 and C1. Using the values shown, the output has an amplitude of ±1 V with rise and fall times of 0.5 ms giving a total Zeta duration of 1 ms.

U1C provides an inverted output of U1B. When the reset pin on U3 is triggered, its Q1 output goes low and the short across C1 by one half of the analog switch (U4) is removed. At this time, the output of U1A will be negative and the output of U1B will start to ramp up. During this time, the output of the circuit is taken via the other half of the analog switch from U1B.

When the ramp reaches its positive voltage threshold, the output of U1A goes positive and U1B will start to ramp down. The output of U1A is taken via U2A and U2B (both inverting Schmitt triggers with special input circuitry that protects the input from negative voltages and allows them to level shift) to the clock input of U3. It’s also taken to the analog switch, which now takes the output of the circuit from U1C (which has an output ramping up from the negative maximum to 0 V). At 0 V, the output of U1A will go negative, resulting in the Q1 output of U3 going high and again shorting C1. This produces 0-V output until the next trigger input resets U3 and the cycle is repeated.